1. Field of the Invention
The present invention relates to a probe for monitoring the level of a liquid.
2. The Prior Art
PFTE (polytetrafluoroethylene), because of its resistance to high temperatures and chemical resistance, has been found to be a particularly advantageous material for insulation sheaths of probes. However, PFTE is not completely impermeable, for example, to water vapor. It has been found that water molecules of hot steam under excess pressure diffuse through the insulation sheath of the probe and that moisture gets in this way into the casing of the probe, where the moisture deposits as a film.
In connection with a known probe (German Patent No. 2,744,864), such a film of moisture causes a leakage current between the end of the insulation sheath and the casing of the probe, acting as the probe housing, which current interferes with the function. Particularly, with sensor electrodes that are long on the container side, only a narrow radial gap is provided between the abutment of the spring on the electrode side and the casing of the probe. Thus, in the event of pendulum-like movements of the sensor electrode occurring in the container, a lateral support occurs by way of the abutment. In this case, moisture film in the zone of the abutment also leads to a leakage current that interferes with the probe's functioning. Such a leakage current occurring in connection with the prior art device between the sensor electrode and the probe housing may lead to inaccurate signals by the probe, i.e., to a signal about a level not conforming to the actual level of the liquid.
With another known probe (German Patent No. 3,026,342), provision is made for a discharge duct for moisture penetrating due to diffusion, such duct leading into the atmosphere. However, it has been found in practical application that in this case too, films of moisture and leakage currents lead to error signals developing between the sensor electrode and the casing of the probe.